Microwave heated fluidized bed reactor having stages

ABSTRACT

An improvement in or relating to the fluidized bed-type heating reactor for introducing into the fluidized bed thereof a water-containing substance to be treated such as an uranyl nitrate solution and subjecting the substance to heat treatment. The reactor of the invention is provided with a microwave-generating means for applying microwaves to the fluidized bed and thereby heating the same.

DESCRIPTION

1. Technical Field

The present invention relates to a fluidized bed-type heating reactorand, more specifically, such a fluidized bed-type heating reactor whichis so structured as to receive in the fluidized bed formed therein awatercontaining substance to be heat-treated such as an uranyl nitratesolution for example, and for heat-treatment of the substance, applymicrowaves thereto.

2. Background Art

Fluidized bed-type heating reactors generally comprise a column providedtherein with a dispersion plate, on which formed is a fluidized bed intowhich a substance is introduced for heat-treatment, and for means forheating, they rely on hot air which is supplied under pressure frombelow the dispersion plate.

Where a reactor of the mentioned type is utilized in or for a process inwhich present is a high water-content condition and which as a wholecomprises a process of an enthalpy ruled rate, it is technically and/oreconomically difficult to meet the whole of a required heat quantitysolely by means of hot air, and it therefore is required to provide anadditional or auxiliary heating means.

One example of such enthalpy-ruled rate processes is the denitrationreaction process that constitutes a part or portion of reprocessing ofspent nuclear fuels. This reaction process is for converting bypyrolysis an uranyl nitrate solution to uranium oxide with moisture andNO_(x) gas liberated, and the heat quantity required in this case is ofthe order of 2000 Kcal/KgU, approximately. Thus, with fluidized bed-typeheating reactors conventionally employed for the mentioned-typedenitration reaction, employment is made for their heating means of aso-called external heating system which utilizes for example aresistance heater disposed on the outer wall of the reactor. Adifficulty with such external heating system resides in that the supplyof the required heat quantity cannot be made with ease and at a highefficiency. That is to say, according to the heating system in referencea certain limit exists with respect to the heat transfer area, and ittherefore is indispensable to determine and select for employment anoptimal means for supplying the required heat quantity. Particularly,when a scaling-up is contemplated of an existing reactor so as to havethe amount of treatment or the capacity for treatment increased, theselection of heating means is difficult to make. Further, in maintaininghigh the temperature of this portion of the reactor wall whichcorresponds to the location of the fluidized bed so as to secure asufficient heat application as desired, another problem is posedrelating to the anticorrosion of the structuring material of thereactor; also, in accordance with an increase in the heat release towardoutside the reactor, heat loss is increased. Furthermore, to raise highthe temperature of the wall of the reactor tends to produce a difficultyin connection with the particle-size control of UO₃ powder to be formed;besides, it is then likely due to a poor heat conduction attributable toa temporary flow failure of the fluidized bed that the fluidized bedundergoes agglomeration, caking and so forth, whereby the serioustrouble is generated of the reactor being made inoperable.

In view of the above, it may be devised to employ an internal heatingsystem in which the heating means for example a resistance heater isprovided inside the reactor column, but insofar as the substance to betreated contains a radioactive component, unavoidable is to prevent theradioactive substance from being externally leaked, whereby structuraldifficulties are involved in this case again from the viewpoint ofmaintenance and replacement of the heater. In addition, particularly inthe case of generally employed cylindrical fluidized beds, a dimensionallimitation cannot be avoided of the internal heater in order to maintaina good fluidized condition of the bed forming particles, whereby if anincrease is made of the heat transfer area, the effect thereof cannotfully be exhibited.

In view of the foregoing described background art, the present inventorshave conducted extensive studies to arrive at the present invention, theobject of which is to provide such a fluidized bed-type heating reactorwhich is so structured as to receive a water-containing substance to beheat-treated in the fluidized bed thereof and apply microwaves to thefluidized bed so that the substance can be thereby advantageouslyheat-treated.

DISCLOSURE OF THE INVENTION

That is to say, the present invention consists in a fluidized bed-typeheating reactor which is adapted to have a water-containing substancesuch as an uranyl nitrate solution introduced into the fluidized bedthereof and have the introduced substance treated by heating and inwhich a microwave-generating means is housed for applying microwaves tothe fluidized bed to thereby have it heated. By such arrangementaccording to the invention, the substance to be heat-treated in thefluidized bed can be extremely advantageously heated. Heat-treatmenttermed herein means such a treatment by which such as dehydration,denitration, oxidation and so forth are effected by heating a substanceto be treated.

BRIEF DESCRIPTION OF THE DRAWINGS

Several figures of the accompanying drawings illustrate embodiments ofthe present invention, in which:

FIG. 1 represents a front view of a cylindrical fluidized bed-typeheating reactor;

FIG. 2 is a view illustrating an example of the microwave introductionpart in the reactor;

FIG. 3 represents a front view of a horizontal fluidized bed-typeheating reactor;

FIG. 4 shows a side elevation of the reactor of FIG. 3;

FIG. 5 represents a front view of a horizontal multi-stage fluidizedbed-type heating reactor; and

FIG. 6 shows a side elevation of the reactor of FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, with reference to the accompanying drawings, the present inventionwill be described in connection with the instance in which the fluidizedbed-type heating reactor according to the invention is utilized in orfor the denitration step constituting a part or portion of reprocessingsteps for spent nuclear fuels.

FIG. 1 shows a front elevation, representing the case in which thefluidized-bed heating reactor is structured in a cylindricalconfiguration, in which the reference numeral 1 denotes a reactor mainbody made of stainless steel, which is structured such that it receivesa substance to be treated in a closed manner therein, permitting noleakage of a radioactive component contained in the substance to betreated, while it at the same time functioning as a microwave shieldingwall as later to be described. This main body 1 has in its lower endportion an aeration pipe line 2 for supplying air into the main body anda discharge pipe line 3 opened on a dispersion plate 5, for dischargingformed matters out of the main body, and at its upper end portion, ablowback filter 4 for removing out of the body and recovering fine dustsentrained by ascending air and also an exhaust pipe line 6 fordeaeration of the interior of the body 1. In the side wall of the mainbody 1, further, there are provided a spray nozzle 7 opened in afluidized bed A formed of powder or dust of UO₃ and also a waveguideduct 8. To the spray nozzle 7, feed pipe lines 9 and 10 through which tofeed an uranyl solution, a substance to be treated, are connected. Withthe waveguide duct 8, an upper end portion thereof is protruded out ofthe main body 1 as shown also in FIG. 2 and, at its top end, providedwith a flange 11, while its lower end is extended to reach the proximityof the surface of the fluidized bed A. Further, at least the portion ofthe duct 8 protruded out of the main body 1 is made of stainless steel,and an arrangement is made such that leakage of radioactive componentsand of radio waves can be effectively checked. In the waveguide duct 8,an irradiation duct 12 is housed, which is structured by aheat-resistant material having a microwave transmittance such as forexample alumina, zirconia or the like. The irradiation duct 12 extendswith its lower end portion into the fluidized bed A, in which an openspace is formed for introducing microwaves in. The reference numeral 13denotes a microwave waveguide connected to a microwave generator notshown. This waveguide 13 is provided with a flange 14, which is coupledto the flange 11 of the waveguide duct 8 through a diaphragm 15 made ofa microwave conductive material such as teflon for example. Thereference character 16 represents a heating device, for example aresistance-type heater, which is provided external to the reactor mainbody 1.

In operation with use of the reactor described above, it may beperformed to charge UO₃ powder in the main body 1 to form a fluidizedbed A, and when air is then introduced into the fluidized bed A throughthe aeration pipe line 2 and also atomized uranyl nitrate solution whichis introduced through the spray nozzle 7, the atomized droplets ofuranyl nitrate solution become attached on surfaces of the UO₃ powderparticles. On the other hand, microwaves are then guided into thefluidized bed A through the irradiation duct 12, so that droplets ofuranyl nitrate solution attached on surfaces of the UO₃ powder becomeheated, and the water and nitrate group in the solution are thereforeconverted to steam and NO_(x) gas, respectively. While the exhaust gascontaining steam and NO_(x) gas are permitted to go up within the mainbody 1, it entrains fine dusts of UO₃, therefore it may be subjected tofiltering of the fine dusts for example through a blowback filter suchas indicated at 4 (a filter capable of backwashing) and may then beliberated into air, if necessary further through a device for removingnitrogen oxides. With UO₃ powder particles, on the other hand, theyundergo a size growth or enlargement by the attachment on their surfacesof UO₃ and are then recovered through the discharge pipe line 3. In theabove steps of the operation, a portion of the formed UO₃ peels off theparticle surfaces of the UO₃ powder or becomes independent powderparticles, so that the operation can be performed continuously.

It will be understood that during operation of the above describedreactor according to the present invention, microwaves are not permittedto leak out but are fully effectively utilized and that the radioactivesubstance is completely shielded in the main body 1 by the waveguideduct 8 and the diaphragm 15 and is therefore not likely to leak out.

Further, although the invention is described in the foregoing inconnection with such an instance in which the irradiation duct 12extended into the fluidized bed A is provided in the number of only one,such duct 12 may be provided in a plural number where necessary so as toeffect a uniform heating of the bed 1, or it may be dispensed with and,instead thereof, employment may be made of the alone of the waveguideduct 8. Also, as before described, an external heater such as the oneindicated at 16 may be made a co-use of with the microwave heatingsystem.

FIGS. 3 and 4 illustrate another embodiment of the fluidized-bed heatingreactor according to the present invention, in which while FIG. 3showing a front elevation of the reactor, FIG. 4 shows a side elevationthereof and in which identical reference characters denote identicalelements or members as in FIGS. 1 and 2. This embodiment to beconsidered is particularly suitable for treating a substance (an uranylnitrate solution) in large amounts, in the denitration step forming apart of reprocessing of spent nuclear fuels.

That is to say, with the substance to be treated there exists a criticalsafety problem, and in view thereof, a certain limitation is applicablewith respect to the diametral size or thickness of the reactor column.Thus, if a cylindrical reactor such as shown in FIG. 1 for example isemployed, the height of the reactor column is subject to a limitationand it therefore is required to install a number of reactor columns.However, if, as in the present embodiment, the reactor main body is madecomprising a horizontal one as indicated at 1' and if the fluidized bedis made longer in the lateral direction than in the vertical directionas indicated at A', by this it is feasible to attain an increase in thetreating amount of the substance with use only of a single reactor. Thereference character 20 represents an overflow weir.

FIGS. 5 and 6 together show a modified example of the reactor shown inFIGS. 3 and 4, and while FIG. 5 showing a front view of the modifiedexample, FIG. 6 shows a side view thereof.

In this example, an overflow weir 20' is provided further to the weir 20in FIG. 3, and the fluidized bed A' is divided into two parts A'₁ andA'₂, in the latter of which lateral irradiation ducts 12' are provided.According to this arrangement, the microwave irradiation can beperformed at different degrees of exposure through the duct 12 andthrough the ducts 12'. Since the substance heated in part A'₁ of thefluidized bed can be further heated in part A'₂, there can be made afuller treatment, that is, it is feasible to reduce the amount ofresidual moisture and nitrate group.

As described above, in the reactor device according to the presentinvention, the fluidized bed A is heated by microwaves. Microwaveheating can exhibit a particularly high heating effect toward moistureor water, and it can therefore be highly effectively utilized inautomatically selectively heating a solution spray-supplied into afluidized bed or any water content scattered or otherwise locallypresent in the fluidized bed; further, heating in this case takes placedirectly through absorption of microwaves by the substance being heated,so that it is free of a limitation with respect to geometricalconfigurations of the heat transfer area; and in addition, microwavesare introduced into the fluidized bed according to the presentinvention, so that the fluidized bed can be with ease heated from insidethereof. Thus, various advantages can be brought about according to thepresent invention, as follows:

(1) A water-containing portion in a fluidized bed which forms one ofmain causes of troubles in or with fluidized beds can be automaticallyselectively heated, so that effectively prevented from occurring can beagglomeration, caking or the like of the fluidized bed which is likelywhen operation lacks stability or attributable to an operation failure.

(2) On account of an internal heating system employed, the heat transferarea on the reactor outer wall can be reduced, so that a reduction inthe reactor size and/or a rationalization of the reactor configurationcan be realized, therefore it is feasible to design such a fluidized bedconfiguration by which a fluidization of the bed can be provided instability.

(3) The invention can with ease be applied to horizontal-typefluidized-bed uranium denitration apparatus which are suitable foreffecting a scaling-up for increasing the treatment amount, and in thiscase, selection of the position for installation and the number of themicrowave waveguide can be easified, therefore, the advantage of theinternal heating system can be more fully exhibited.

(4) With the reactor arrangement in which a dispersion plate is mountedin a lower portion within the reactor in a manner of laterally extendingfrom one end to a location adjacent the other end of the reactor, a weiris disposed on the dispersion plate at the above-mentioned locationadjacent the other end, a fluidized bed is formed in the space on thedispersion plate formed between the reactor wall and the weir, andmicrowave irradiation means is disposed in the fluidized bed, it isfeasible to obtain a relatively long time of stay of a substance to betreated in the fluidized bed, and to carry out a continuous operation byway of charging a substance to be treated in a portion of the fluidizedbed located remote from the weir and, while the substance to be treatedis being conveyed toward the weir, subjecting the substance to heatingby microwaves and by taking out the treated substance over the weir;further, the amount of irradiation of microwaves can be suitablyadjusted in proportion to the water content in the substance to betreated, so that it is feasible to treat a substance in mass.

(5) By providing a plurality of weirs in the reactor arrangement recitedin paragraph (4) above, it is feasible to provide a plurality of dividedfluidized bed portions in the reactor, and it is then feasible to carryout heating at varied degrees in separate fluidized bed portions inaccordance with various possible conditions of the divided bed portions.

(6) Fluidized bed reactors can with ease be so arranged as to have alarger diametral part in their upper portions and a smaller diametralpart in their lower portions, so that an appropriate space can be withease secured for introducing microwaves in, whereby it is feasible toput a microwave power of a large capacity in a substance introduced intothe fluidized bed.

(7) The microwave heating of the internal heating system may be employedeither singly or in combination with an external heating system such asby a resistance-type heater, so that selection of the heating system canbe made with ease.

(8) A forward end portion of the irradiation duct is so disposed as tolie in the fluidized bed, so that heating can be effected in a uniformmanner and at a high efficiency in the fluidized bed.

CAPABILITY OF EXPLOITATION IN INDUSTRY

Whereas the present invention has been described in connection with theinstance in which the fluidized bed-type heating reactor embodying theinvention is applied as a denitration reactor for reprocessing spentnuclear fuels, it will be apparent that the reactor according to thepresent invention can be made various other applied uses and that inconnection with the described specific structural features of theinvention, various changes and modifications can with ease be madewithout departing from the spirit of the invention.

We claim:
 1. A fluidized bed-type heating reactor of the form in whichwater-containing nuclear fuel material is introduced into a fluidizedbed so as to undergo a heat-treatment, characterized in that adispersion plate is provided in a lower portion within a reactor mainbody laterally extending from one side to a point in the vicinity of theopposite side of the reactor main body; at least two weirs are mountedabove the dispersion plate, the at least two weirs being step-wisereduced in height to a discharge opening of the reactor main body, thefluidized bed being partitioned by said at least two weirs into aplurality of laterally continuously arranged fluidized-bed portions;means for delivering microwaves to each of the fluidized-bed portionscomprising a waveguide duct which houses one end of an irradiation duct,the other end of which is disposed in respective fluidized bed portionsso as to apply microwaves to thereby heat the fluidized-bed portions todifferent temperatures and have the nuclear fuel material heat-treated.2. A fluidized bed-type reactor as set forth in claim 1, characterizedin that externally at a wall portion of said reactor main bodysubstantially corresponding to the location of the fluidized bed, thereis a heater provided for heating said wall portion.
 3. A fluidizedbed-type heating reactor as set forth in claim 1, characterized in thatin an upper portion of said reactor main body there is a filterincorporated for removing and recovering fine particles entrained byair.
 4. A fluidized bed-type heating reactor as claimed in claim 1,characterized in that said reactor main body has in an upper portionthereof a large diametral part having a lower end portion of a reduceddiameter and has a small diametral part formed from the bottom of saidlower end portion and in that said fluidized bed is formed in said smalldiametral part.
 5. A fluidized bed-type heating reactor as set forth inclaim 1, characterized in that means is incorporated for introducing asubstance to be treated into said fluidized bed, said means comprising aspray device having a spray opening opened in said reactor main body. 6.A fluidized bed-type heating reactor as set forth in claim 1,characterized in that said watercontaining substance to be treated is anuranyl nitrate solution.